While the salmon is cooking prepare the sauce. Blanch the spinach and parsley by placing them in a pot of boiling water for 30 seconds (be sure not to overcook; the spinach and parsley should retain their bright green color). Remove promptly from the hot water and strain the liquid out. In a blender or food processor, combine the spinach, parsley, olive oil, mustard, apple cider vinegar and tarragon and puree.